Conventionally, nickel-cadmium cells have been the main cells used as secondary cells for memory-backup or sources for driving AV (Audio Visual) and information devices, particularly personal computers, VTRs (video tape recorders) and the like. Lately, non-aqueous electrolyte secondary cells have been drawing a lot of attention as a replacement for the nickel-cadmium cells because non-aqueous electrolyte secondary cells have advantages of high voltage, high energy concentration, and displaying excellent self-dischargeability. Various developments of the non-aqueous electrolyte secondary cells have been performed and a portion of these developments has been commercialized. For example, more than half of notebook type personal computers, cellular phones and the like are driven by the non-aqueous electrolyte secondary cells.
Carbon is often used as a cathode material in the non-aqueous electrolyte secondary cells, and various organic solvents are used as electrolytes in order to mitigate the risk when lithium is produced on the surface of cathode, and to increase outputs of driven voltages. Further, particularly in non-aqueous electrolyte secondary cells for use in cameras, alkali metals (especially, lithium metals or lithium alloys) are used as the cathode materials, and aprotic organic solvents such as ester organic solvents are ordinarily used as the electrolytes.
However, although these non-aqueous electrolyte secondary cells exhibit high performance, they have the problem described below with safety.
Namely, alkali metals (especially, lithium metals or alloys) that are used as negative electrode materials for the non-aqueous electrolyte secondary cells are extremely highly-active with respect to water. Therefore, for example, when the non-aqueous electrolyte secondary cell is imperfectly sealed, and water enters therein, a problem occurs in that negative electrode materials and water are reacted with each other, whereby hydrogen is generated to ignite the cell. Further, since a lithium metal has a low melting point (about 170° C.), when a large current is suddenly flown into a cell during a short circuit or the like, and an excessive amount of heat is generated, an extremely high danger occurs in which the cell is molten or the like. Moreover, due to the generation of heat, when the electrolyte is evaporated or decomposed to generate gas, a danger occurs in which the cell explodes and ignites.
In order to solve the aforementioned problems, when temperature ascends and pressure inside the cell rises during the short circuit or overcharge of a cylindrical cell, for example, a method having a mechanism in which an excessive amount of current is prevented from flowing into the cylindrical cell by a break of electrode terminals at the same time when the safety valve is operated (Nikkan Kogyo Shinbun, Electronic Technology, Vol. 39, No. 9, 1997).
However, it is not necessary ensured that the mechanism operates normally all the time. When the mechanism does not operate normally, a possibility of danger still remains in which more heat is generated by the excessive current thus causing the cell to ignite.
Thus, development has been a high demand of an excellent non-aqueous electrolyte secondary cell which does not need a safety mechanism such as a safety valve but is able to mitigate risks due to evaporation, decomposition, or ignition of the electrolyte, and exhibit fundamentally high safety and which also exhibit stability as good as that in conventional non-aqueous electrolyte secondary cells, good resistance to deterioration, and good electrochemical characteristics.
Further, development has been required of non-aqueous electrolyte secondary cells which have excellent low-temperature characteristics because cell properties must be maintained for a long period of time even under low-temperature conditions such as in the regions or season in which temperature is low.
On the other hand, a non-aqueous electrolyte electric double layer capacitor is a condenser making use of electric double layers formed between polarizable electrodes and electrolytes.
The electric double layer capacitor is different from a cell in which a cycle of an oxidation-reduction reaction accompanied by substance movements is a charging/discharging cycle in that a cycle for electrically absorbing, on electrode surfaces, ions from electrolytes is a charging/discharging cycle. For this reason, the electric double layer capacitor is more excellent in instant charging/discharging properties than those of a cell. Repeatedly charging/discharging the capacitor does not deteriorate the instant charging/discharging properties. Further, in the electric double layer capacitor, since excessive charging/discharging voltage does not occur during charging/discharging, simple and less expensive electric circuits suffice for the capacitor. Moreover, the capacitor has more merits than the cell from the viewpoints that it is easy to know a remaining capacitance in the capacitor, and the capacitor has endurance under conditions of a wide range of temperature of from −30° C. to 90° C., and the capacitor is pollution-free. Consequently, the electric double layer capacitor is in the spotlight as a new energy storage product that is kind to the global environment.
The electric double layer capacitor is an energy storage device comprising positive and negative polarizable electrodes and electrolytes. At the interface at which the polarizable electrodes and the electrolytes come into contact with each other, positive and negative electric charges are arranged so as to face one another and be separated from one another by an extremely short distance to thereby form an electric double layer. The electrolytes play a role as ion sources for forming the electric double layer. Thus, in the same manner as for the polarizable electrodes, the electrolytes are an essential substance for controlling fundamental properties of the energy storage device.
As the electrolytes, aqueous-electrolytes, non-aqueous electrolytes, or solid electrolytes are conventionally known. However, from a viewpoint of improvement of energy concentration of the electric double layer capacitor, the non-aqueous electrolyte in which a high operating voltage is enabled has particularly been in the spotlight, and practical use thereof is progressing.
A non-aqueous electrolyte is now put to practical use in which solutes such as (C2H5)4P.BF4 and (C2H5)4N.BF4 were dissolved in highly dielectric solvents such as carbonic acid carbonates (e.g., ethylene carbonate and propylene carbonate), γ-butyrolactone, and the like.
However, these non-aqueous electrolytes have been a problem in that when a non-aqueous electrolyte electric double layer capacitor is heated and ignited, an electrolyte is ignited, flames are combusted to spread over the surfaces thereof, resulting in a high risk.
Further, these non-aqueous electrolytes has been a problem in that, as the non-aqueous electrolyte electric double layer capacitor generates heat, the non-aqueous electrolyte that uses the organic solvent as a base is evaporated or decomposed to generate gas. Due to the generated gas, the non-aqueous electrolyte electric double layer capacitor may explode or ignite thus causing the electrolyte to catch fire, flames are combusted to spread over the surfaces thereof, resulting in a high risk.
Lately, as the practical use of the non-aqueous electrolyte electric double layer capacitors has been developed, application thereof to electromobiles, hybrid cars, or the like has been expected, whereby a requirement for safety of the capacitors has been increasing more and more.
Accordingly, development of a non-aqueous electrolyte electric double layer capacitor has been increasing day by day which has a property in which risks due to evaporation, decomposition or ignition of the non-aqueous electrolyte can be mitigated and which also exhibits various excellent properties: flame retardancy when the non-aqueous electrolytes are ignited, self-extinguishability or flame retardancy, and accordingly high safety, and deterioration resistance. Further, in accordance with high evolution of technology, development of a non-aqueous electrolyte electric double layer capacitor has been a high demand in which various properties such as low internal resistance, high electric conductivity, and long term stability are accomplished at the same time.
Further, there has been a demand for development of a non-aqueous electrolyte electric double layer capacitor which is excellent in low-temperature characteristics because electric characteristics must be maintained for a long period of time even under low-temperature conditions such as in regions or season in which temperature is low.